1. Field of the Invention
The present invention relates to a thermal printer for feeding a heat transfer ribbon and a paper toward and between a platen and a thermal head while the heat transfer ribbon remains laid on top of the paper so that the former is held by the latter, followed by performing printing on the paper by ink which is impregnated in the heat transfer ribbon and is molten by the thermal head when the heat transfer ribbon and the paper are allowed to pass between the platen and the thermal head.
2. Prior Art
In case of printing, e.g. a bar code, etc. on a paper P using a thermal printer as illustrated in FIG. 8, large blank spaces are liable to appear on the paper P between printed portions of the bar codes.
In such a case, if the bar code, etc. are continuously printed on the paper P while the heat transfer ribbon remains laid on top of the paper, the heat transfer ribbon is allowed to pass between the platen and the thermal head while it is not used for printing at the portions corresponding to the blank spaces on the paper P. If the ratio of the blank spaces to the printed portions is high, the heat transfer ribbon is consumed soon, which leads to increase of the running cost.
There is a thermal printer having a mechanism for feeding the heat transfer ribbon in the normal direction toward the thermal head and also in the direction opposite to the normal direction independently of the normal direction wherein the heat transfer ribbon having the portion which is not subjected to printing is reversely fed to the position close to the printing position by the thermal head after a given time lapses every time the printing is completed.
Such a printer is disclosed in, e.g. Japanese Utility Model Publication No. 62-41809. This is explained more in detail with reference to FIG. 12 wherein a heat transfer ribbon 1 can be fed together with or independently of a paper P in the normal direction as denoted at the arrow A or in the direction opposite to the normal direction.
The paper P to be printed thereon is pressed against the outer periphery of a platen 2 by a friction roller 3 at one portion thereof while it remains extended around the platen 2.
The heat transfer ribbon 1 unwound from a supply roller 4 is allowed to pass an ink sheet feed roller 6 and a separation roller 7 which are respectively disposed at both sides of the platen 2 and is wound around a winding roller 5.
A thermal head 8 contacts the platen 2 at the printing position of the paper P and turnably attached to one end of a rod 11 at one end thereof (right side in FIG. 12) and the rod 11 is rotatably attached to a rotary portion of a rotary solenoid 12 at the other end of the rod 11.
Accordingly, when the rotary solenoid 12 rotates, the rod 11 lowers from the position in FIG. 12 to thereby swing the thermal head 8 in the direction as denoted at the arrow B so that the pressing force of the thermal head 8 against the platen 2 can be reduced or the thermal head 8 can be moved away from the platen 2.
The ink sheet feed roller 6 is driven by a stepping motor, not shown. When the ink sheet feed roller 6 is reversely rotated by the stepping motor, it can feed the heat transfer ribbon 1 in the direction opposite to the direction of the arrow A.
Accordingly, when there appears a large blank space between the printed portions, the heat transfer ribbon I and the paper P are fed in the direction of the arrow A whereby the printing is performed on the paper P, thereafter they are still fed in the direction of the arrow A and then the heat transfer ribbon 1 is separated from the paper P. Successively, the stepping motor is reversely rotated to return the heat transfer ribbon 1 by the length corresponding to the blank space so that the heat transfer ribbon can be effectively used.
There is another thermal printer which is disclosed in Japanese Patent Publication No. 2-59068 as illustrated in FIG. 13 wherein a heat transfer ribbon 1 is reversely fed independently of a paper.
In this thermal printer, a paper P to be printed thereon is fed toward and between a thermal head 14 and a platen 15 by way of a plurality of paired feed rollers 13 and 13 and the heat transfer ribbon 1, which is unwound from a supply roller 9 and wound around a winding roller 10, is laid on top of the paper P before the heat transfer ribbon 1 and the paper P reach the thermal head 14. The paper P and the heat transfer ribbon 1 which is laid on top of the paper P are fed toward and between the thermal head 14 and the platen 15 so that the former is held by the latter wherein ink impregnated in the heat transfer ribbon 1 is molten by the thermal head 14 to thereby transfer ink onto the paper P so as to complete the printing.
The thermal head 14 contacts the heat transfer ribbon 1 at a heat sensitive print head 17 which is held integrally by a head block 18. The head block 18 is turnably supported by a supporting shaft 19 so as to turn the entire thermal head 14 in the direction as denoted at the arrow C. After the heat transfer ribbon 1 is unwound or drawn out from the supply roller 9 and extended around a tension arm 21, it is allowed to pass a pinch roller 23 which is pressed by a return feed roller 22 provided with a one way clutch capable of shifting counterclockwise as illustrated in FIG. 13 and resiliency of a spring, not shown, and thereafter it is extended around a guide roller 24 and held between the heat sensitive print head 17 and the platen 15.
Successively, the heat transfer ribbon 1 is extended around a separation roller 25 and thereafter is allowed to pass a pinch roller 27 which is pressed by a return feed roller 26 provided with a one way clutch capable of shifting counterclockwise as illustrated in FIG. 13 and resiliency of a spring, not shown, and thereafter it is extended around a tension arm 28 and then wound around the winding roller 10.
If there appear large blank spaces between the printed portions, the heat transfer ribbon 1 is fed in the direction opposite to the normal feeding direction, i.e. rightward in FIG. 13 every time the printing is completed so as to effectively use and save the heat transfer ribbon 1.
That is, the heat transfer ribbon 1 and the paper P are laid on top of another just before they reach the heat sensitive print head 17 and they are fed toward and between the heat sensitive print head 17 and the platen 15 where the printing is performed and thereafter they remain continuously fed in the same direction. When the printed portion on the paper P where an image is transferred thereon is allowed to pass the separation roller 25 and the heat transfer ribbon 1 is separated from the printed portion on the paper P, a motor, not shown, is reversely rotated to rotate reversely the return feed roller 22 to thereby return the heat transfer ribbon 1 by the length corresponding to the blank space where it is not used for printing, thereafter the printing is repeated in a predetermined timing.
However, since the thermal printer as illustrated in FIG. 12 is structured that the heat transfer ribbon 1 unwound from the supply roller 4 is fed toward the thermal head 8 by the ink sheet feed roller 6 in the direction of the arrow A, there is such a problem that the heat transfer ribbon 1 is not always stably stretched at a given tension between the supply roller 4 and the thermal head 8 since the tension to be applied to the heat transfer ribbon 1 is increased as the outer diameter of the heat transfer ribbon 1 wound around the supply roller 4 is reduced.
This is described more in detail. According to the thermal printer as illustrated in FIGS. 12 and 13, the tension is always applied to the heat transfer ribbon 1 positioned between the supply roller and the thermal head so as to remove the slack on the heat transfer ribbon 1 so that the wrinkle is prevented from generating on the heat transfer ribbon 1.
A mechanism for applying tension is, for example, illustrated in FIG. 14. The mechanism applies a given tension to the heat transfer ribbon 1 which is positioned between the supply roller 4 and the thermal head 8 by applying frictional load to a ribbon supply shaft 29 wherein the frictional load generated by a frictional load generating gear 31 is transmitted to a friction transmitting gear 32 which is integrated with the ribbon supply shaft 29 meshing with the frictional load generating gear 31 whereby the frictional load is transmitted to the ribbon supply shaft 29.
The frictional load generating gear 31 is rotatably supported by a shaft 33 which is fixed perpendicular to a printer side plate 30 as illustrated in FIG. 15 and has a friction generating pad 36 which is integrally attached to one surface thereof. A spring presser 34 is screwed into a tip end side of the shaft 33 and a compression coil spring 35 is mounted between the spring presser 34 and the frictional load generating gear 31. The frictional load generating gear 31 is pressed against the printer side plate 30 by the resiliency of the compression coil spring 35 whereby the friction generating pad 36 is pressed against the printer side plate 30 so that the frictional load generating gear 31 is rotated with a given frictional load.
In such a mechanism, if the frictional load is applied, e.g. to the supply roller 4 as illustrated in FIG. 12 to thereby apply the tension to the heat transfer ribbon 1 which is positioned between the supply roller 4 and the thermal head 8, the following relation is established which is represented by the equation of T=F/R where F is friction to prevent the rotation of the ribbon supply shaft 29, T is the tension applied to the heat transfer ribbon 1 and R is a radius of the heat transfer ribbon 1 which is wound around the supply roller 4. Accordingly, this equation shows that the tension T is increased as the radius R of the heat transfer ribbon 1 wound around the supply roller 4 is reduced. The tension to be applied to the heat transfer ribbon 1 is varied since the radius of the transfer ribbon 1 reduced as the transfer ribbon is used so that the heat transfer ribbon 1 can not be stably stretched at a given tension between the supply roller 4 and the thermal head 8.
In the mechanism as illustrated in FIG. 15, the friction generating pad 36 is worn as time lapses to thereby vary the coefficient of friction so that the frictional load to be applied to the ribbon supply shaft 29 is varied, which leads to the problem of variation of the tension to be applied to the heat transfer ribbon as time lapses.
There is a so-called ON DEMAND operation in the thermal printer which means that both the heat transfer ribbon 1 and the paper P can be fed in the direction opposite to the feeding direction thereof at the same. That is, as illustrated in FIG. 14, the paper P, after it was printed, is cut by a cutter 38 which is positioned understream the thermal head 8 in the paper feeding direction, and thereafter the cut paper P is fed reversely so that the cut portion of the paper P is returned together with the heat transfer ribbon 1 to the printing position where the printing is to be performed by the thermal head 8 and at the same time the next printing is performed on the printing position, i.e., blank space on the cut print paper P so as to save the heat transfer ribbon 1 and the paper P.
In the operation of the so-called ON DEMAND, the heat transfer ribbon 1 is likely to slack and generate the wrinkle thereon upstream the thermal head in the heat transfer heat feeding direction, i.e. at the left side in FIG. 14 when the heat transfer ribbon 1 is returned together with the paper P.
In such a thermal printer, a coil spring 37 is provided between the ribbon supply shaft 29 and the friction transmitting gear 32 in which the ribbon supply shaft 29 rotatably engages and one end of the coil spring 37 is fixed to the ribbon supply shaft 29 wherein the other end thereof is fixed to the friction transmitting gear 32 as illustrated in FIG. 16 (components which correspond to those of FIG. 15 are denoted at the same numerals).
With such an arrangement, when the heat transfer ribbon 1 is unwound around fed from the supply roller 4 by the feeding force of the platen 2, the ribbon supply shaft 29 is rotated in the direction opposite to the winding direction of the coil spring 37 by a given amount to thereby generate turning effort.
The coil spring 37 is twisted until the turning effort of the ribbon supply shaft 29 balances with friction generated by friction between the friction generating pad 36 of the frictional load generating gear 31 and the printer side plate 30. Thereafter, the friction surface of the friction generating pad 36 slips so that the frictional load generating gear 31 idles and the friction transmitting gear 32 engaging with the frictional load generating gear 31 rotates to thereby rotate the ribbon supply shaft 29 while the coil spring 37 remains twisted.
When the heat transfer ribbon 1 is returned together with the paper P owing to the ON DEMAND operation, the twist of the coil spring 37 is returned by the amount of returning of the heat transfer ribbon 1 and the paper P whereby the heat transfer ribbon 1 which is liable to slack is again wound around the supply roller 4, so that the slack of the heat transfer ribbon 1 is removed and the wrinkle is prevented from generating on the heat transfer ribbon 1.
However, the provision of the coil spring 37 increases the number of parts and requires the troublesome assembly thereof, which leads to poor assembling operation thereof.
In the thermal printer as illustrated in FIG. 13, since the heat transfer ribbon 1 which is unwound and fed from the supply roller 9 is held by the return feed roller 22 and the pinch roller 23 until it reaches the thermal head 14, if the mechanism to apply a given tension to the heat transfer ribbon 1 as illustrated in FIG. 15 is provided in the return feed roller 22 or pinch roller 23 the tension is directly applied to the heat transfer ribbon 1 from the return feed roller 22 or pinch roller 23. Although the variation of the tension has been occurred as the outer diameter of the heat transfer ribbon 1 wound around the supply roller is varied when the frictional load is applied to the ribbon supply shaft 29, such variation of the tension does not occur so that the heat transfer ribbon 1 can be always held between the thermal head 14 and the pinch roller 23 at a given tension.
However, the structure to hold the heat transfer ribbon 1 by the rollers at both sides thereof increases the number of the rollers, which leads to much time and labor for assembling the thermal printer and to poor maintenance thereof at the time of replacing the parts thereof with other parts.
Furthermore, in the conventional typical thermal printer, the paper is liable to skew if the thickness thereof is varied. It is preferable that the paper hardly skews even if the kind of paper such as the thickness thereof is varied but it is not performed easily by the conventional arrangement of the thermal printer.
In the arrangement of the thermal printer as illustrated in FIG. 13 for holding and feeding the paper and the feed transfer ribbon by a plurality of paired rollers respectively at both sides thereof, there is such a problem that the stable feeding force can not be obtained since the paper and the heat transfer ribbon can not be held thereby with a uniform pressing force extending in the width direction of the paper and the heat transfer ribbon when a plurality of paired rollers are not disposed in accurately parallel with each other in the axial directions thereof owing to the variation of the accuracy of the parts.